1. Technical Field of the Invention
The present invention relates to earth boring bits, and more particularly to those having rotatable cutters, also known as cones.
2. Description of Related Art
Reference is made to U.S. Pat. No. 3,397,928 to Galle, the disclosure of which is hereby incorporated by reference. Reference is still further made to U.S. Pat. No. 3,765,495 to Murdoch, the disclosure of which is hereby incorporated by reference.
Earth boring bits with rolling element cutters have bearings employing either rollers as the load carrying element or with a journal as the load carrying element. The use of a sealing means in rock bit bearings has dramatically increased bearing life in the past fifty years.
Early seals for rock bits were designed with a metallic Belleville spring clad with an elastomer, usually nitrile rubber (NBR). The metallic spring provided the energizing force for the sealing surface, and the rubber coating sealed against the metal surface of the head and cone and provided a seal on relatively rough surfaces because the compliant behavior of the rubber coating filled in the microscopic asperities on the sealing surface. Belleville seals of this type were employed mainly in rock bits with roller bearings. The seal would fail due to wear of the elastomer after a relatively short number of hours in operation, resulting in loss of the lubricant contained within the bearing cavity. The bit would continue to function for some period of time utilizing the roller bearings without benefit of the lubricant.
A significant advancement in rock bit seals came when o-ring type seals were introduced. These seals, as disclosed by Galle, were composed of nitrile rubber and were circular in cross section. The seal was fitted into a radial gland formed by cylindrical surfaces between the head and cone bearings, and the annulus formed was smaller than the original dimension as measured as the cross section of the seal. The squeeze of the seal was defined as the percentage reduction of the cross section from its original state to the compressed state. Murdoch disclosed a variation of this seal by elongating the radial dimension which, when compared to the seal disclosed by Galle, required less percentage squeeze to form an effective seal. This was referred to in the art as a high aspect ratio seal. Several other minor variations of this concept have been used, each relying on an elastomer seal squeezed radially in a gland formed by cylindrical surfaces between the two bearing elements.
Over time, the rock bit industry has moved from a standard nitrile material for the seal ring, to a highly saturated nitrile elastomer for added stability of properties such as thermal resistance and chemical resistance.
Reference is now made to FIG. 1 which illustrates a prior art configuration for an earth boring bit. FIG. 2 illustrates a close-up view of the prior art configuration focusing on the area of the sealing system 2 and journal bearing 11 associated with a rotating cone 4 and a shaft 5 of the bit head 1. An o-ring seal 6 is squeezed between a cylindrical cone sealing surface 9 and a cylindrical head sealing surface 7. The term “cylindrical” in this context refers to a surface that is parallel to an axis of cone rotation. An inner radial cone surface 8 is provided on the grease side of the seal, while an outer radial head surface 10 is provided on the drilling fluid side of the seal. The term “radial” in this context refers to a surface extending away from the axis of cone rotation. In this illustrated implementation, the radial surfaces 8 and 10 are normal (i.e., perpendicular) to the axis of cone rotation. It will be noted that the sealing pressure is between surfaces 7 and 9.
Reference is now made to FIG. 3 wherein there is shown another prior art seal. In this case the seal is a high aspect ratio seal. It will be noted that the seal 6 is squeezed between the cylindrical cone sealing surface 9 and the cylindrical head sealing surface 7. An inner radial (normal) cone surface 8 is provided on the grease side of the seal, while an outer radial (normal) cone surface 10 is provided on the drilling fluid side of the seal. The normal surfaces 8 and 10 are required to stabilize the seal (for example, prevent motion, buckling, twisting) when in operation. It will also be noted that surface 10 is located on the cone 4 (not the head as in FIG. 2).
FIGS. 2 and 3 both illustrate the use of a journal bushing 12 positioned between the rotating cone 4 and the cantilever bearing shaft 5 in the area of the journal bearing 11.
With the configurations illustrated in FIGS. 1, 2 and 3, the seal 6 is directly exposed to heat generated by the journal bearing 11 associated with the shaft 5 and rotating cone 4. Exposure to this heat during operation of the rock bit is known to change the mechanical properties of the elastomer used for the o-ring seal 6. A reduced seal life results from this heat exposure. A need exists to protect the o-ring seal from this heat exposure.